1. Field of the Invention
The present invention relates to a method of determining the defect size of an evaluation pattern formed on an evaluation mask and used to evaluate the defect detection sensitivity of a mask defect inspection system, and also relates to a method of creating a sensitivity table.
2. Background Art
In the manufacturing process of semiconductor devices, photomasks and reticles (hereinafter referred to collectively as “masks”) are used as original artwork patterns for forming a circuit pattern on a substrate (also referred to as a “wafer”). A mask is typically a transparent glass substrate having a pattern of a light shielding material (e.g., Cr) formed thereon. Charged particle beam writing apparatuses such as electron beam apparatuses, which provide superior resolution, have been used to manufacture such masks.
If a mask has a defect, the image of that defect is transferred to the substrate during the photolithographic printing process, resulting in reduced yield of semiconductor devices. To address this problem, mask defect inspection systems have been used to inspect masks for defects. There are two known mask defect inspection methods: the die-to-die inspection method and the die-to-database method. In the die-to-die inspection method, optical images of two separate identical patterns on a mask are compared to each other. In the die-to-database inspection method, on the other hand, an optical image of a mask is compared to a reference image derived from the writing data (or CAD data) that was used to manufacture the mask.
Evaluation masks (also referred to as programmed defect masks) are used to evaluate the defect detection sensitivity of mask defect inspection systems. One known type of evaluation mask is the sensitivity mask, which has formed thereon a plurality of different-size programmed defects (also referred to as “simulated defects”) of each defect type. The sensitivity mask may be inspected for defects a plurality of times by a mask defect inspection system, and the results of the plurality of inspections may be used to create a sensitivity table showing the defect detection rate of the mask defect inspection system for each programmed defect, making it possible to evaluate the defect detection sensitivity of the mask defect inspection system. Further, such a sensitivity table may be created periodically to manage the state of the mask defect inspection system.
The recent trend toward finer circuit patterns requires accurate control of the critical dimension (CD) of patterns. To achieve this, a mask defect inspection system is used to inspect the patterns on the mask to find ones whose critical dimension deviates from its design value by a large amount. Such defective patterns are said to be CD defects (or dimensional defects). Japanese Laid-Open Patent Publication No. 2002-296762 discloses a method of detecting a CD defect. In order to evaluate the CD defect detection sensitivity of mask defect inspection systems, each sensitivity mask has formed thereon a plurality of programmed CD defects (serving as evaluation patterns) whose CDs differ from the design value of the CD of the reference pattern (or desired pattern) by different amounts.
In order to accurately evaluate the defect detection sensitivity of a mask defect inspection system, it is necessary to accurately determine the defect size of each evaluation pattern included in the sensitivity mask used. According to a conventional defect size determining method, the sensitivity mask has formed thereon a reference pattern which has a reference critical dimension (or design dimension). With this arrangement, the dimension of the reference pattern and the corresponding dimension (i.e., critical dimension) of each evaluation pattern described above are measured, and the difference between the two measured values is regarded as the defect size of the evaluation pattern.
The above reference pattern and evaluation patterns are formed on the sensitivity mask through processes such as writing by an electron beam writing apparatus, developing, and etching. Generally, the reference pattern and the evaluation patterns on the sensitivity mask are spaced from each other. It should be noted that two supposedly identical patterns (i.e., patterns whose dimensions have the same design value) formed on the sensitivity mask may be slightly different in dimension from each other if they are not at the same location, because of the difference between their writing positions and because of process variations. This means that the defect size of an evaluation pattern determined by the conventional defect size determining method as described above includes, in addition to the “true” defect size (or designed defect size), dimensional variation due to the difference between the writing positions of the evaluation pattern and the reference pattern and due to process variations. This prevents accurate determination of the defect size of evaluation patterns, making it impossible to accurately evaluate the defect detection sensitivity of the mask defect inspection system.
The present invention has been made in view of the above problems. It is, therefore, an object of the present invention to provide a defect size determining method of defect detection sensitivity evaluation patterns, capable of accurately determining the defect size of an evaluation pattern on an evaluation mask used to evaluate the defect detection sensitivity of mask defect inspection systems.